In dairy plants and other food plants, process lines are used to conduct process fluids between upstream pumps, such as centrifugal pumps, and downstream devices of various sorts, such as spray heads, level controls, or other similar or dissimilar devices. The process fluids may be liquids, slurries, aerated liquids, or other substantially liquid, pumpable products, as exemplified by milk, other dairy products, and other food products.
Commonly, in such a plant, a cleaning cycle is employed, in which process lines are cleaned in place, drained, and purged with clean air. Thus, when such lines are returned to active service, such lines are empty of process fluids or other liquids. Therefore, as explained below, certain problems tend to arise when such lines are returned to active service.
Commonly, motor-driven pumps, such as centrifugal pumps, are used to pump process fluids through process lines at flow rates depending upon flow resistances of such lines, flow resistances of downstream devices, and other factors. One problem that tends to arise is that, if a centrifugal pump begins to pump a process fluid into a process line when the process line is empty of the process fluid or any other liquid, the process fluid tends to be initially pumped at a flow rate that is much higher than the flow rate associated with normal operation of the centrifugal pump. The motor driving the centrifugal pump is thus overloaded. These conditions of higher flow rate and pump motor overload tend to prevail until the process fluid fills the process line between the centrifugal pump and a downstream device.
Another problem arises because a line surge is produced when a process fluid is pumped into a process line when the process line is not filled with the process fluid. As a mass of the process fluid cannot be instantly decelerated when the mass reaches a downstream device or any prior restriction in the process line, and as a line surge tends to create abnormally high pressures in the process line, a line surge may damage the process line, the downstream device, line hangers, walls supporting such hangers, or associated equipment. The line surge problem is exacerbated when caustic cleaning solutions or hot food products leak unexpectedly from damaged equipment.
Prior efforts to alleviate these problems have taken three different approaches, which have proved to be very costly to implement. A first approach has been to install a system enabling a slow start (which may be also called a "soft" start) of a motor driving a centrifugal pump. A second approach has been to use a variable-speed motor to drive a centrifugal pump at a slower speed until the process fluid fills the process line and at a higher speed after the process fluid has filled the process line. A third approach has been to install a pneumatically powered valve with a positioner at a discharge end of a centrifugal pump. None of these approaches has proved to be entirely satisfactory.
Therefore, there has been a need, to which this invention is addressed, for a simpler, less costly approach to alleviating these problems, particularly but not exclusively in a dairy plant or another food plant where good sanitation is of paramount importance.